-LRB- CNET -RRB- -- Apple likes glass . A lot . You can tell that much from its many retail stores , and in its products where glass is used in displays , notebook trackpads , and both the front and back of the iPhone 4 .

That last place , though is what might be causing the company some headaches .

That 's according to a report today from Gdgt , which says that slip-on cases -- the kind that cover the back and sometimes front of the iPhone 4 , have been the cause of serious cosmetic damage with the backside of the iPhone . Dirt and other loose bits of debris from your pocket end up in that space .

Over time , that can lead to a shattered backside as small scratches grow to become large cracks that travel across the back of the device , much like a ding on a windshield .

This has become a big enough problem , the report says , that Apple 's engineers have been hard at work in `` a quiet lockdown , '' testing various third-party cases to see how widespread the problem is , and presumably to make sure it does not happen with future iterations of the device .

The news comes at an especially interesting time given the recent expiration of Apple 's free iPhone 4 case program -LRB- which included Apple 's no closed-back bumper -RRB- , and the reported beginnings of mass production for a CDMA version of the iPhone said to be coming to Verizon in early 2011 .

If Apple is planning to bring any physical design changes to that version of the iPhone , a back that 's susceptible to cracking could very well be holding up that process .

This would n't be the first time cracks have cropped up on Apple 's hardware , or the iPhone line for that matter .

Both the Mac G4 Cube , which was introduced in 2000 , and the iPhone 3G experienced reports from users that the outsides were developing `` hairline surface cracks . ''

Similar blemishes had appeared on white versions of the company 's MacBook notebooks , which Apple reportedly began acknowledging and repairing early last year .

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Slip-on cases have caused serious cosmetic damage to backside of the iPhone 4

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Over time , small scratches grow to become large cracks like a windshield

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Apple 's engineers have been hard at work in `` a quiet lockdown '' to fix problem

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This is n't the first time cracks have cropped up on Apple 's hardware